An exposure apparatus is one type of precision stage device that is commonly used to transfer images from a mask to a substrate in various manufacturing processes. A typical exposure apparatus usually includes one or more stages or plants for retaining and moving the mask and/or the substrate. One example of an exposure apparatus is a lithography device called a wafer scanner or wafer stepper, which performs one of the many essential steps in the manufacturing process of integrated circuits (ICs). The wafer scanner or stepper includes a reticle stage that retains a reticle, i.e., mask, and a wafer stage that retains a semiconductor wafer, i.e., substrate. During the manufacturing process, a control system directs a control signal to one or more motors coupled to the reticle stage and/or wafer stage to generate forces that position one or both of the stages relative to an illumination source and optical assembly with high precision.
As the circuitry on ICs become smaller, the precision required for controlling movement of the stages increases proportionally. In order to meet specifications that are currently on the order of nanometers, control systems require careful design. Precise positioning of the wafer and the reticle relative to the optical assembly is critical to the manufacture of high density, semiconductor wafers.
During stage control, a variety of disturbances can lead to errors in stage movement. For example, the stage may develop a following or positioning error quantified as the difference between an intended or desired trajectory of the stage and an actual trajectory of the stage at a specified time. In other cases the stage may experience periodic or repeating vibrations or ripple forces. As a result, precision in the manufacture of the semiconductor wafers can be compromised, potentially leading to issues in production quality and throughput. In some cases, one or more effects of a stage motor can contribute to errors in stage movement and less than desirable performance in general.
Although existing control systems reduce unwanted disturbances and corresponding movement errors to some degree (e.g., through vibration damping systems), there is significant room for improvement. Along with the ever-present desire to manufacture smaller ICs and other micro-devices comes a requirement for even more precise stage movement with smaller disturbances and movement errors. Thus, there is a need for control methods and systems that can improve the accuracy in the positioning of one or more stages of a precision stage device. Further, there is a need for control systems and methods that can accurately compensate for one or more effects of a stage motor on a stage control system.